Ab initio periodic Hartree-Fock calculations along with corrections for correlation effects have been carried out to study the crystal structure of alpha-MoO3. The nature and energetics of bonding within the solid have been investigated and related to the geometry of the crystal. (MoO3)(x) chains formed by MoO4 tetrahedra sharing common O corners were shown to be the basic structural units of the solid. Simple Mo ... O electrostatic interactions between these chains lead to the formation of the natural layers of MoO3 and establish a 6-fold coordination around Mo atoms. The (100) surface of alpha-MoO3 solid contains pentacoordinated Mo atoms, brought about by the breaking of interchain interactions, and possessing a Lewis acid character. The adsorption of H2O and CO molecules on these Mo sites was found to be of, essentially, electrostatic nature restoring a bulklike environment around Mo atoms.